Vacuum table conveyors are well known where a substantially uniform vacuum is applied across the entire surface of the table through a multiplicity of small, regularly spaced openings. A major drawback of such tables is their lack of versatility. It may be difficult or impossible to apply a sufficiently satisfactory vacuum to a plastic film web to be conveyed across the table, if the web is so small that significant numbers of the vacuum openings through the table are exposed. Alternatively, it may be difficult to develop or maintain a substantially uniform vacuum between an edge of such a web, near a large number of exposed vacuum opening and a center portion of the web, surrounded by covered openings.
It would therefore be beneficial to provide an air table conveyor capable of being used with plastic film webs of differing sizes.
It would also be advantageous, in certain situations, to be able to apply different air pressures at different locations such as a first, higher vacuum, where the conveyor initially receives the film from an adjoining feeder, and a lower vacuum where the film is removed from the conveyor.
Alternatively, it would be advantageous to be able to apply a greater vacuum near the edges of a plastic film on such a conveyor and a lesser vacuum on the center of the plastic film to reduce loading on the conveyor.
It would also be advantageous to be able to apply positive and/or negative air pressures to selected areas of such a conveyor. Positive air pressure refers to a pressure greater than one atmosphere while negative air pressure is one less than one atmosphere (i.e. vacuum).
It would also be advantageous to be able to hold the film on the conveyor by vacuum and/or electrostatic attraction.